1. Field of the Invention
This invention relates to grain oriented electromagnetic steel sheets having a very low iron loss and a method of producing the same, and more particularly to a grain oriented electromagnetic steel sheet, in which the surface of base metal in this sheet after finish annealing is smoothened up to a surface roughness having a center-line average roughness Ra of not more than 0.3 .mu.m through mechanical polishing before the formation of insulating coating, and a method of smoothening the steel sheet through such a mechanical polishing, particularly, mechanical polishing with free abrasive grains or elastomeric polishing member containing abrasive grains.
2. Related Art Statement
The grain oriented electromagnetic steel sheets are mainly used as a core material for transformers and other electrical machineries, so that they are more strongly demanded to have excellent magnetic properties, particularly a very low iron loss (exemplified by W.sub.17/50 value).
As to such demands, there have hitherto been developed a method of highly aligning secondary recrystallized grains of &lt;100&gt; orientation in the steel sheet into the rolling direction thereof, a method of reducing impurities contained in a final product and the like, whereby it was possible to reduce the W.sub.17/50 value of the sheet having a thickness of 0.23 mm to about 0.9 W/kg.
However, it strongly tends to request electrical machinery and apparatus having less power loss on the border of energy crisis since several years. For this purpose, it becomes demanded to develop grain oriented electromagnetic steel sheets having much lower iron loss as a core material for these machineries and apparatuses.
In general, as the fundamental technique for reducing the iron loss of the grain oriented electromagnetic steel sheet, there are mainly known metallurgical methods such as method of increasing the Si amount, method of thinning the thickness of the product, method of finely dividing the secondary recrystallized grains, method of reducing the impurity amount, method of highly aligning the secondary recrystallized grains of (100)[001] orientation and the like. However, these techniques already arrive at the limit in view of the existing production technique, so that further improvement is very difficult. Even if the improvement is somewhat observed, the effectivenesss of improving the iron loss is still lacking at the present.
In Japanese Patent Application Publication No. 54-23,647, there is proposed a method of finely dividing the secondary recrystallized grains by forming secondary recrystallization preventing regions in the steel sheet surface, but this method can not be said to be practical because the control of secondary recrystallized grain size is unstable.
Furthermore, a technique of reducing the iron loss by introducing microstrain into the surface of the steel sheet after the secondary recrystallization with steel sheets for use in a ball-pointed pen to conduct magnetic domain refinement is disclosed in Japanese Patent Application Publication No. 58-5,968, and a technique of conducting magnetic domain refinement for the reduction of the iron loss by irradiating a laser beam to the surface of the final product in a direction substantially perpendicular to the rolling direction at an interval of few mm to introduce high dislocation density regions into the surface layer of the steel sheet is disclosed in Japanese Patent Application Publication No. 57-2,252. Moreover, a technique of reducing the iron loss by introducing microstrain into the surface layer of the steel sheet through discharge working to conduct magnetic domain refinement is proposed in Japanese Patent laid open No. 57-188,810.
These three techniques attempt the reduction of iron loss by introducing micro plastic strain into the base metal surface of the steel sheet after the secondary recrystallization to conduct magnetic domain refinement and are alike practical and excellent in the effect of reducing iron loss, but have a drawback that the effect by the introduction of micro plastic strain is undesirably diminished by subsequent strain relief annealing treated after punching, shearing or winding of steel sheet or by heat treatment such as baking of coating layer. Moreover, when the introduction of micro plastic strain is carried out after the coating, it is required to conduct reapplication of insulating coating for maintaining the insulation property, which largely increases the steps such as strain giving step, reapplication step and the like and brings about the increase of the cost.
Besides, Japanese Patent Application Publication No. 52-24,499 discloses that the surface of the silicon steel sheet after the finish annealing is pickled to remove oxides from the surface thereof and rendered into a mirror finished state by subjecting to a chemical polishing or electrolytic polishing to improve the magnetic properties and particularly reduce the iron loss.
In this case, however, the chemical polishing or electrolytic polishing for the mirror finishing is required, so that the cost becomes very high, and consequently such a polishing has a conspicuous difficulty in the actual application to industrial process and is not yet adopted in the mass production.
Furthermore, it is difficult that a phosphate series tension coat usually used as a tension insulating coating for the grain oriented silicon steel sheet is closely formed on the mirror finished surface of the sheet without damaging good magnetic properties obtained by the smoothening of the surface.
If it is intended to cut the expenses by replacing the above expensive polishing step with a mechanical polishing step using, for example, grindstone or the like, the remaining strain through the mechanical polishing is given to the silicon steel sheet, resulting in the considerable degradation of iron loss, so that such a mechanical polishing is impossible to be put into practical use.